1. Field of the Invention
A first aspect of the invention relates to an organic electroluminescent device. This first aspect also relates to a substrate for an organic electroluminescent device.
A second aspect of the invention relates to a multilayer display device. It is particularly suitable, but by no means limited, to multilayer display devices fabricated using organic light emitting diodes.
2. Related Technology
First Aspect
Organic electroluminescent devices are known, for example from PCT/WO/13148 and U.S. Pat. No. 4,539,507. Such devices generally comprise a substrate 2, a first electrode 4 disposed over the substrate 2 for injecting charge of a first polarity; a second electrode 6 disposed over the first electrode 4 for injecting charge of a second polarity opposite to said first polarity; an organic light-emitting layer 8 disposed between the first and the second electrodes; and an encapsulant 10 disposed over the second electrode 6. In one arrangement shown in FIG. 1, the substrate 2 and the first electrode 4 are transparent to allow light emitted by the organic light-emitting layer 8 to pass therethrough. Such an arrangement is known as a bottom-emitting device. In another arrangement shown in FIG. 2, the second electrode 6 and the encapsulant 10 are transparent so as to allow light emitted from the organic light-emitting layer 8 to pass therethrough. Such an arrangement is known as a top-emitting device.
Variations of the above-described structures are known. The first electrode may be the anode and the second electrode may be the cathode. Alternatively, the first electrode may be the cathode and the second electrode may be the anode. Further layers may be provided between the electrodes and the organic light-emitting layer in order to aid charge injection and transport. The organic material in the light-emitting layer may comprise a small molecule, a dendrimer ora polymer and may comprise phosphorescent moieties and/or fluorescent moieties. The light-emitting layer may comprise a blend of materials including light emitting moieties, electron transport moieties and hole transport moieties. These may be provided in a single molecule or on separate molecules.
By providing an array of devices of the type described above, a display may be formed comprising a plurality of emitting pixels. The pixels may be of the same type to form a monochrome display or they may be different colors to form a multicolor display.
A problem with organic electroluminescent devices is that much of the light emitted by organic light-emitting material in the organic light-emitting layer does not escape from the device. The light may be lost within the device by scattering, internal reflection, waveguiding, absorption and the like. This results in a reduction in the efficiency of the device. Furthermore, these optical effects can lead to low image intensity, low image contrast, ghosting and the like resulting in poor image quality.
A further problem with organic electroluminescent devices is that of achieving intense, narrow band-width emission so as to improve the color purity of emission.
One way of solving the aforementioned problems is to utilize microcavity effects within a device.
A microcavity is formed when the organic light-emitting layer is disposed between two reflecting mirrors, one of which is semitransparent. The photon density of states is modified such that only certain wavelengths, which correspond to allowed cavity modes, are emitted with emission intensity being enhanced in a direction perpendicular to the layers of the device. Thus emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced through the semitransparent mirror and emission at wavelengths away from the resonance is suppressed.
A weak microcavity is achievable using a standard device structure of the type described above. For example, in a bottom-emitting device such as that illustrated in FIG. 1, a metal cathode 6 is generally utilized along with an ITO anode 4. The metal cathode is highly reflective while the ITO is substantially transparent but is weakly reflective. This weak reflectivity of ITO can result in a weak cavity effect. However, ITO provides a poor cavity not only because it is weak, but also because its refractive index is very variable over the visible spectrum. This results in variable performance with difference wavelengths and viewing angles.
In light of the above, it is known to alter the structure of organic electroluminescent devices in order to provide an improved microcavity effect.
U.S. Pat. No. 6,861,800 discloses several modified arrangements. In one arrangement, illustrated in FIG. 3a of U.S. Pat. No. 6,861,800, the ITO anode is replaced with a semitransparent silver anode. A stronger microcavity is thus formed between the semitransparent silver anode and a reflective silver cathode when compared with an arrangement using an ITO anode as illustrated in FIG. 3b of U.S. Pat. No. 6,861,800. In fact, the microcavity effect of ITO is so low that in U.S. Pat. No. 6,861,800 such an arrangement is described as having no microcavity.
One problem with replacing the anode electrode with a stronger mirror is that the electrical properties of the device will be altered.
In U.S. Pat. No. 6,861,800 an alternative arrangement shown in FIG. 3c has been proposed in which a Quarter Wave Stack (alternatively known as a Distributed Bragg Reflector) is disposed between the ITO anode and transparent substrate. A QWS is a multi-layer stack of alternating high and low index dielectric layers which may be tuned so as to have very high reflectance, very low transmittance and practically zero absorbance over a given range of wavelengths. Such an arrangement provides a very strong microcavity.
WO 00/76010 also discloses the use of QWS between a substrate and an anode of a bottom-emitting device. As in U.S. Pat. No. 6,861,800, ITO is not considered to contribute a cavity effect in the device and is described as transmissive.
In “A. Dodabalapur et al., Physics and applications of organic microcavity light emitting diodes, J. Appl. Phys. 80 (12), 1996, 6954-6964” an arrangement is disclosed in which a QWS and a filler layer are provided between a transparent substrate and an ITO anode. The filler layer is of variable thickness so as to provide a number of cavities tuned to different colors. As in the aforementioned documents, the ITO anode is not considered to contribute a cavity effect in the device and is described as transmissive.
One problem with arrangements which utilize a QWS is that the microcavity can be too strong. Although the absorbance of the QWS is practically zero, the layers of material between the QWS and the cathode do absorb light. Thus, because light is trapped in the cavity until it enters a mode which can pass through the low transmittance QWS, absorption of light by layers of material within the cavity becomes significant. Furthermore, the QWS results in a very narrow emission resulting in a narrow viewing angle and the color of the emission changes with viewing angle. Additionally, a QWS is complicated and expensive to manufacture requiring the deposition of a number of additional layers and increases the thickness of the finished device.
Another problem with arrangements which utilize a QWS is that the microcavity can be too large. Utilizing a QWS, the net effect of reflection from the stack of layers occurs approximately in the middle of the stack. As such, the distance between the net reflection from the QWS and the reflecting electrode forming the other side of the microcavity is large. The mode spacing is thus small as the mode spacing of a microcavity is inversely proportional to the size of the microcavity. With a large microcavity, while some parts of the spectrum are enhanced, other parts of the spectrum are reduced in intensity due to the small mode spacing allowing many modes to be accessed. As a result, although spectral narrowing with an increase in intensity of certain wavelengths can be achieved with a QWS, the overall enhancement of light output may be minimal due to reduction in intensity in other parts of the spectrum.
In “T Shiga, Design of multiwavelength resonance cavities for white organic light-emitting diodes, J. Appl. Phys. 93 (1), 2003, 19-22” an organic electroluminescent device is disclosed in which two microcavities are provided. Such an arrangement is illustrated in FIG. 2 of this document. A first cavity is formed between an ITO anode and a cathode. A second cavity is formed between the cathode and a layer of high refractive index dielectric material spaced apart from the ITO anode by a spacer layer of a low refractive index dielectric material. Such an arrangement is designed to produce two emission peaks in different areas of the visible spectrum which mix to form a white emission.
One problem with this arrangement is that it is not suitable for improving the color purity of emission as it is specifically directed to producing a white emission. Another problem with this arrangement is that, as stated previously, the ITO layer does not form a good cavity.
Second Aspect
Displays fabricated using organic light emitting diodes (OLEDs) provide a number of advantages over other flat panel technologies. They are bright, colorful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colors which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Other passive displays include segmented displays in which a plurality of segments share a common electrode and a segment may be lit up by applying a voltage to its other electrode. A simple segmented display need not be scanned but in a display comprising a plurality of segmented regions the electrodes may be multiplexed (to reduce their number) and then scanned.
FIG. 12 shows a vertical cross section through an example of an OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 12). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500 nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are available from Corning, USA. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.
A substantially transparent hole transport layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene) from Bayer AG of Germany. In a typical polymer-based device the hole transport layer 106 may comprise around 200 nm of PEDOT; a light emitting polymer layer 108 is typically around 70 nm in thickness.
These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited as disclosed in, for example, EP 0880303. Such wells define light emitting areas or pixels of the display.
Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved or enhanced through the use of cathode separators (not shown in FIG. 12).
The same basic structure may also be employed for small molecule devices.
Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.
To illuminate the OLED power is applied between the anode and cathode, represented in FIG. 12 by battery 118. In the example shown in FIG. 12 light is emitted through transparent anode 104 and substrate 102 and the cathode is generally reflective; such devices are referred to as “bottom emitters.”
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a TV picture, to give the impression of a steady image.
Referring now to FIG. 13, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of FIG. 12 are indicated by like reference numerals. As shown the hole transport 106 and electroluminescent 108 layers are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).
The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore often encapsulated in a metal can 111, attached by UV-curable epoxy glue 113 onto anode metal layer 104, small glass beads within the glue preventing the metal can touching and shorting out the contacts. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UV light for curing.
Conventional metal cathodes 110 are inherently reflective. As a consequence, a problem experienced with prior OLED devices is that the displayed image is degraded by the reflection of ambient light from the cathode 110. This reflected light combines with the light that is being intentionally emitted by the OLED device when forming an image. This gives rise to an image that suffers from poorer contrast and worse color depth than desired.
One method used to reduce the amount of reflected light is to apply a circular polarizer coating on the outside of the substrate 102. However, the use of a circular polarizer necessitates increased power consumption and reduces contrast.
Another method to reduce the amount of reflected light is to use a contrast-enhancing stack available from Luxell of Ontario, Canada under the name “Black Layer”. As shown in FIG. 14, the Black Layer is a multilayer structure 202, 204, 206 (rather than a single layer as its name might otherwise suggest) that replaces the traditional metal cathode 110. This is a destructive-optical-interference multilayer filter, which is incorporated between the opaque electrode and the organic stack. A thin (ca. 20 Å) metal layer 202 (e.g. barium) is first applied next to the electroluminescent layer 108, and then a semi-transparent layer 204 phase-changing layer incorporating a transparent conducting oxide (e.g. a Cr:SiO layer 100 nm thick or a SiO based layer) is deposited on the thin metal layer. Finally, a thick layer of reflective metal 206 (e.g. aluminium) forming the cathode is deposited on the semi-transparent layer 204. Incident ambient light passing through the substrate 102 is reflected by both the thin metal layer 202 and the thick metal layer 206. The thickness of the intermediate semi-transparent layer 204 is selected such that light reflected from the thin metal layer 202 is 180° out of phase compared with light reflected from the thick metal layer 206. Light rays reflected from the two metal layers combine destructively, thereby reducing the amount of reflected ambient light. Other destructive interference systems utilising a half-wavelength spacing layer located on the cathode side of an OLED are disclosed in, for example, WO 00/35028 and JP 08-008065. A display using such an interference system requires less power than one with a circular polarizer, and provides higher contrast.
The inventors have found that a further contrast reducing reflection results from reflection of light at the interface between the anode 104 and the substrate 102. Due to the relatively high refractive index of a typical ITO anode 104 compared with the relatively low refractive index of a typical glass substrate 102, 10% of the ambient light intensity can be reflected from the anode/substrate boundary. There is a desire to reduce the intensity of light reflected from this boundary to the order of 1%, to provide a comparable level of reflection to that which would be achieved by circular polarizers (which, as mentioned above, are disadvantageous in that they necessitate increased power consumption and reduce contrast).
It may be possible to utilize light reflected from a layer such as a Black Layer located on the cathode side of an OLED to destructively interfere with light reflected at the anode/substrate boundary. However, this requires adjustment for the effects of thickness, refractive index, etc of the intervening organic layer and as such represents a complex method of minimising reflections at the anode.